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The hardness of a mineral is one of the most valuable characteristics for mineral identification. When rockhounding, the hardness test is simple to administer at home or in the field.
One key to identifying mineral species is to test its hardness. Mineral hardness tests can be performed at home or on a rockhounding field trip. There are necessary tools for express hardness testing: A fingernail, a Cooper coin (penny or 1,2.5 cents euro), a shard of glass, and a knife blade.
You already have two of the tools required for the Mineral Hardness Test! These are your fingernails and the coins in your pocket. We will cover the Mohs Scale and how to utilize it to rapidly detect minerals in dairy products. There’s no need for something special, like a unicorn horn. By the way, geologists love the hardness test the most!
If you want to check out the best Mohs hardness test kits, you can find them only by clicking here (Amazon link).
Why is Mineral Hardness Important?
Mineral hardness indicates how resistant minerals are to scratching. Every mineral has its hardness. So, being able to do a hardness test provides a quick technique to identify minerals.
Mineral hardness is essential since it indicates minerals’ chemical composition and structure. Knowing the mineral hardness helps to narrow down a list of possible minerals during sample identification. The hardness tests can clearly distinguish between two remarkably similar-looking materials.
How Do You Determine the Hardness of a Mineral?
The hardness test doesn’t require any complex equipment. It can be easily held even at home with the tools everybody has. Stay tuned, and you will receive step-by-step instructions.
To determine the hardness of a mineral – do the Mohs Hardness Test. Scratch the mineral with a tool with known hardness. Observe if a tool leaves scratches or not. If it does – the mineral is softer. If it doesn’t – the mineral is harder. Compare the result to the Mohs Scale and find a possible mineral.
How to Do a Mohs Hardness Test?
The Mohs hardness test is just an official name of the hardness test. Introduced in 1822, the scale originated when Friedrich Mohs chose ten minerals and assigned numbers to them based on the relative ease or difficulty with which another could scratch one. His studies resulted in the scale below, which is still used today.
The Mohs hardness test is done by scratching a mineral sample with a tool or other mineral with known hardness. If quartz (7 on the Mohs scale) cannot scratch your sample, your mineral is harder than 7. If your sample cannot scratch a piece of glass, your mineral’s hardness is lower than 5.
The Mohs Scale ranks minerals and gem’s hardness from 1 (least hard – talc) to 10 (hardest of all – diamond).
The Mohs hardness scale:
| Mineral | Hardness |
|---|---|
| Talc | 1 |
| Gypsum | 2 |
| Calcite | 3 |
| Fluorite | 4 |
| Apatite | 5 |
| Feldspar (sometimes indicated as Orthoclase) | 6 |
| Quartz | 7 |
| Topaz | 8 |
| Corundum (ruby and sapphire) | 9 |
| Diamond | 10 |
It’s vital to understand that the Mohs scale is not a linear scale. Instead, it ranks minerals on a relative scale based on their scratch hardness, so although corundum (ruby or sapphire) is a 9, a diamond at 10 is almost 5 times harder (according to Vickers Absolute Hardness Scale) and 3 times harder (according to Knoops Absolute Hardness Scale)!
According to the Mohs Scale, only a diamond can scratch a diamond. Corundum can scratch itself, topaz (8), quartz (7), and anything softer. Topaz can scratch itself, quartz (7), and anything lower on the scale.
Important notification!
The hardness of a stone indicates the stone’s resistance to scratching or how the surface of the mineral will respond to contact with a sharp point. This differs from a mineral’s toughness, defined by how well a mineral can survive an impact or resist breaking, chipping, or cracking.
Consequently, a diamond can be easily shattered with a hammer since the diamond is brittle. However, the diamond cannot be scratched with the same hammer. Toughness and hardness are different physical characteristics.
TIP: A Mohs hardness test is one of the most useful tricks that should be in all rockhounds’ playbooks. Check out the best test kits in the article below:
3 Best Mohs Scale Test Kits: Test the Hardness of Your Gemstones
How Do You Test the Hardness of a Mineral (at home)?
The hardness of minerals can be easily tested at home. You will need some simple tools with known hardness and a pinch of patience.
You will need your fingernail, copper wire or coin, a piece of glass, a knife blade, and a steel file to perform the hardness test. All of them have known hardness comparable to the Mohs Scale. Scratch your mineral with the tools described above. The harder tool leaves a scratch on the softer mineral.
Here is a step-by-step instruction on how to test the hardness of a mineral at home:
- Prepare something resistible to protect the floor or a table surface where you will conduct a test. Your mineral can be hard, but tabletop can not.
- Put your unknown mineral sample and fix it firmly.
- Scratch your mineral with a fingernail. If you see a scratch on the mineral surface, the mineral hardness is below 2.5. Compare this hardness with the Mohs Scale. Your mineral is very soft, and most probably, it is talc or gypsum.
- If there is no visible scratch when using a fingernail, consider using copper wire or a copper coin, such as a penny or a 1, 2, or 5-cent euro coin. Cooper’s relative hardness is 3 on the Mohs scale. It can scrape minerals with a hardness level below 3. Fluorite may be scratched when pressure is exerted on it.
Important note! Nowadays, it is widely claimed that modern coins are mainly made of zinc, such as the penny, or are coated with a layer of copper, as is the case with the 1, 2, and 5-cent euro coins. Therefore, it is advisable to utilize copper wire
- A test with a piece of glass is essential since its hardness lies in the center of the Mohs scale. It distinguishes minerals based on whether they are in the lower or upper half of the Mohs Scale. In the case of glass, testing it in the other direction is preferable. Take the mineral and try scratching a piece of glass. If a mineral can scratch it, then the hardness of the unknown mineral is more than 5. It could be feldspar, quartz, topaz, corundum, or diamond. If a mineral cannot scratch a piece of glass, it is softer than 5.
- A common knife has a very similar hardness to glass. So, it can also be used to differentiate whether a mineral is softer or harder than 5.
- A steel file is harder (6-6.5) than a knife blade (5-5.5). This implies that a steel file will scratch feldspar but not quartz.
- Quartz is also an excellent tool for hardness testing because practically every mineral enthusiast has a quartz crystal in their collection. So quartz will scratch anything below a 7. If quartz cannot scratch an unknown material, we have some excellent news for you! You likely have topaz, corundum (ruby and sapphire), or a diamond.
- Diamonds can scratch any mineral. Diamond is the hardest. But remember – it is not the toughest.
Important note: Please use caution when using jewelry containing diamonds for the hardness test since they can chip due to the pressure exerted.
Another important note! As you can see, different minerals and gemstones have different hardness. Therefore, it is recommended that minerals and jewelry are to be stored separately to avoid scratching.
BTW: Do you want to know more about rock and mineral identification? The books listed below are the best ones you can find on the internet (Amazon links):
- Smithsonian Handbooks: Rocks & Minerals
- Gemstone & Crystal Properties (Quick Study Home)
- Ultimate Explorer Field Guide: Rocks and Minerals (National Geographic Kids)
Some tips for a successful hardness test:
- The scratch test is not the same as the rub test. You don’t have to rub rocks with glass. This could be confusing because it’s difficult to tell if it’s a scratch or bits of the tool you used. It should be one straight scratch.
- It’s also a good idea to wipe off a scratch to remove the white chalky bits left behind. This will make it easy to see a scratch.
- Additionally, using a magnifying glass to look at a scratch can be very helpful sometimes. Sometimes, it’s tough to distinguish between a scratch and a white sign.
- Minerals or only very coarse-grained rocks can be used for the scratch test. It is hard to test gemstones that are less than 1 cm in size because most rocks are made up of them. Also, different minerals in rock do not all have the same hardness. To test the natural strength of stones, you need extra skills.
- One material, kyanite, has different levels of hardness in other places. So, the hardness changes depending on the way you test it. It’s 4.5 to 5 when tested along the length of a crystal but 6.5 to 7 when tested across the short dimension of a crystal. So, different material directions can be used to test the hardness.
TIP: Licking rocks and minerals in geology is a common practice to define some kind of minerals and, of course, a bunch of fun. Find out how to lick your rocks like a pro in the article below:
Licking Rocks in Geology: Why & How (Answered by Geologist)
What Tools Are Used to Test a Mineral Hardness?
Tools for a mineral hardness test are very simple. Everybody has it. You must know their equivalent hardness to the Mohs Scale to perform the test. Further, you will find a clue.
Common tools to test a mineral’s hardness are a fingernail, Cooper wire or coins (penny or 1,2,5 cents euro), a knife, a piece of glass, and a steel file. Additionally, a set of minerals with a known hardness can be used. For example, quartz crystal, whose hardness is 7, is also very helpful.
Here is a table of various tools’ hardness equivalent to the Mohs scale:
| Tool | Mohs Hardness | Comment |
|---|---|---|
| Fingernail | 2 – 2.5 | A fingernail will scratch talc and gypsum but will not scratch calcite. |
| Cooper wire or coin | 3 | Copper cannot be scratched by talc, gypsum, and calcite but will be scratched by fluorite, apatite, feldspar, and quartz. |
| A piece of glass | 5 – 5.5 | A piece of glass scratches everything up to apatite. But will be scratched by feldspar, quartz, topaz, corundum (ruby and sapphire), and diamond. |
| Knife blade | 5 – 5.5 | A knife blade cannot scratch quartz and higher. |
| Steel file | 6 – 6.5 | A steel file can differentiate between feldspar and quartz. A steel file scratches feldspar but cannot scratch quartz. |
TIP: If you want to check out the best Mohs hardness test kit, I recommend you find it here (Amazon link).
Important note!
The above-mentioned materials, especially a knife blade and a piece of glass, can have different hardnesses based on their make-up, flaws, and the way they were made.
For a more accurate test, the Mohs Mineral Hardness Test Kit is better. It has 9 minerals from the Mohs Scale or Hardness Pick Set, but not diamond. The Hardness Pick Set is a set of measured picks that look like pencils. They are made of alloys that were carefully chosen to match the hardness of the rocks on the Mohs scale. It helps make a test of strength more accurate.
Conclusion
Testing mineral hardness at home can be easily performed according to the Mohs hardness test. The Mohs Scale is ten minerals and assigned numbers based on the relative ease or difficulty with which one can be scratched by another, where 1 (least hard – talc) to 10 (hardest of all – diamond).
The Hardness test compares what material can scratch another or not. Only a diamond can scratch a diamond. Corundum can scratch itself, topaz (8), quartz (7), and anything softer. Topaz can scratch itself, quartz (7), or anything lower on the scale.
Equivalent tools can be useful if you don’t have minerals from the Mohs Scale. If a tool leaves a scratch on the unknown mineral, the mineral is softer than the tool.
Some common tools:
- fingernail (2 – 2.5),
- cooper wire or coins (penny or 1,2,5 cents euro) (3),
- knife blade (5 – 5.5),
- piece of glass (5 – 5.5), and
- steel file (6 -6.5)
are used for a Hardness test as they are equivalent to some values in the Mohs Scale.
TIP: If you want to check out the best Mohs hardness test kit, click here (Amazon link).
TIP: Can you imagine 300 kg of rock moving in a desert and leaving tracks behind without any help? Sounds ridiculous. But it has a rational explanation. Find it out in the article below:
Sailing Stones Explained: Why & How Do They Move? (7 Facts)